The invention relates to devices equipped with a colour prism, such as colour line scan cameras, colour scanners and other such optoelectronic devices.
Colour line scan cameras are used in industry and in official applications necessitating a high frame rate, high-quality colour separation and a high-resolution image. Colour line scan cameras employ two basic technologies for producing colour separation. In the first method, different wavelength ranges are provided with separate sensors whose shape and order vary but which are characterized in that colour separation is carried out in the actual sensor element, typically using filters. In devices according to the first method, sensors of different wavelength ranges are usually positioned in closely-spaced rows of their own. A typical device according to the first method is provided with adjacent rows of sensors for blue, green and red colour channels. The second method is based on a colour separation prism which, utilizing selectively reflecting surfaces and total reflection, separates each colour onto an image surface of its own. Line image sensors placed on these image surfaces allow to be located on top of one another optically.
The problem with the devices according to the above-described first method, wherein colour separation is carried out in the actual sensor element, is inaccurate colour registration since images of different colours are not stored at exactly the same point in time from the same source. A further problem is that images of different colours are not stored from the same direction nor at the same optical distance.
A colour line scan camera equipped with a colour prism enables an image of each colour of a certain object to be stored at the same moment, from the same direction and at the same optical distance, but since in this method in addition to air, glass of the prism is also located between an objective lens and an image sensor, optical characteristics deteriorate by a factor which depends on the length of a distance travelled by light in the glass. The colour separation prism deteriorates the optical characteristics of a conventional objective lens because conventional objective lenses are designed assuming that the medium in front of the image surface is air. Attempts have been made to solve the problem by providing colour line scan cameras with objective lenses to enable characteristics deteriorating the image quality of the prism to be reduced. This method enables quite good results to be achieved when an objective lens is designed for a particular application and when the f-number of the objective lens is allowed to deteriorate. A problem with this method is that the custom-made objective lens makes the colour line scan camera much more expensive. The colour line scan camera is often a part of a more comprehensive computer vision system, in which case using the above-disclosed method also makes the entire computer vision system more expensive since a deteriorated f-number requires more efficient lightning.
In addition to colour line scan cameras, the above-described prior art problems also relate to all other corresponding opto-electronic devices, such as colour scanners.